Rotating electrical machine and drive system of cage induction motor

ABSTRACT

A rotor conductor upper portion positioned near to an outer periphery of a rotor has a trapezoidal, cross sectional shape, a rotor conductor lower portion positioned nearer to a center of the rotor than the rotor conductor upper portion has a rectangular, cross sectional shape, and the rotor conductor upper portion has a height h 1 =27 mm or more in the case where rotor conductors are made of brass, and a height h 1 =7 mm or more in the case where the rotor conductors are made of copper.

This application is a continuation-in-part of application Ser. No.10/775,227, filed on Feb. 11, 2004, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a rotating electrical machine, of whicha rotor comprises a cage conductor, and a drive system of a cageinduction motor.

JP-A-7-231630 discloses a rotating electrical machine, in which rotorconductors of a cage induction motor are sectioned along a planeperpendicular to a shaft to comprise a lower portion enlarged stepwisein width as compared with an upper portion positioned near to an outerperiphery of a rotor. Also, “Transformer/induction machine/AC commutatormotor” published by Institute of Electrical Engineers, 6th edition, Nov.21, 1983, pages 311 to 315 (FIG. 3.63, FIG. 3.64, and FIG. 3.67)discloses a rotating electrical machine, in which rotor conductors havethe same cross sectional shape as that in JP-A-7-231630 and the crosssectional shape of the rotor conductors over a whole height thereoftapers linearly toward an outer periphery of a rotor to be trapezoidalin shape.

The rotor conductors disclosed in JP-A-7-231630 have an invertedT-shaped cross section such that the upper portions of the rotorconductors positioned near to the outer periphery of the rotor are smallin width and the lower portions are enlarged stepwise in width.Therefore, high resistance is generated at the starting of a rotatingelectrical machine, so that starting characteristics of the rotatingelectrical machine can be improved. On the other hand, since the rotorconductor is enlarged stepwise in width and the rotating electricalmachine is sharply reduced in secondary resistance, the rotatingelectrical machine is reduced in torque. Also, coming-out of the rotorconductors due to centrifugal forces is restricted only by the steps ofthe rotor conductors. Therefore, centrifugal forces concentrate on thesteps and the rotor conductors are in some cases broken by friction.Further, since the rotor conductors are sharply changed in width, a hightechnique is needed for processing of the rotor conductors in the casewhere a drawn material is used for the rotor conductors.

Meanwhile, with the rotating electrical machine of“Transformer/induction machine/AC commutator motor”, the cross sectionalshape of the rotor conductors tapers over a total height thereof towardthe outer periphery of the rotor to be trapezoidal in shape, so thathigh resistance is generated at the starting of the rotating electricalmachine and a starting characteristics of the rotating electricalmachine is improved. Since the rotor conductors are trapezoidal-shaped,however, a height thereof and a width of a bottom thereof are determinedby restriction in strength, there is little freedom in design, and astarting characteristics and a steady-state characteristics of therotating electrical machine are not balanced in some cases.

It is an object of the invention to provide a rotating electricalmachine having starting characteristics of low starting current and highstarting torque, and a good balance between a starting characteristicsand a steady-state characteristics.

It is a further object of the invention to provide a drive system of acage induction motor, in which alternating current with a commercialfrequency remaining intact is supplied directly to the induction motorto realize a characteristics of low starting current and high startingtorque, and a characteristics of high efficiency and high power factor.

Other objects and features of the invention will be made apparent fromthe following description of the embodiments.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the invention, the rotor conductors comprise a rotorconductor upper portion, which is positioned near to an outer peripheryof a rotor, and a cross sectional shape of which tapers continuouslytoward the outer periphery of the rotor, and a rotor conductor lowerportion, which is contiguous to the rotor conductor upper portion to bepositioned nearer to a center of the rotor than the rotor conductorupper portion, and a cross sectional shape of which is made rectangularto have substantially the same width as that of a bottom of the rotorconductor upper portion.

Here, it is desired that a cross sectional shape of the rotor conductorupper portion be trapezoidal to taper linearly toward the outerperiphery of the rotor.

In another aspect of the invention, brass is used to make the rotorconductors and the rotor conductor upper portion has a height of notless than 27 mm.

In a further aspect of the invention, copper is used to make the rotorconductors and the rotor conductor upper portion has a height of notless than 7 mm.

In a still further aspect of the invention, the rotor conductor upperportion is made of brass and the rotor conductor lower portion is madeof copper.

Owing to these features, a rotating electrical machine is achieved toeasily meet demanded steady-state characteristics such as efficiency,power factor, and so on while restricting a starting current andensuring a starting torque.

In a further aspect of the invention, the rotor conductors comprise arotor conductor upper portion, which is positioned near to an outerperiphery of a rotor, and a cross sectional shape of which taperscontinuously toward the outer periphery of the rotor, and a rotorconductor lower portion, which is contiguous to the rotor conductorupper portion to be positioned nearer to a center of the rotor than therotor conductor upper portion, and a cross sectional shape of which ismade rectangular to have substantially the same width as that of abottom of the rotor conductor upper portion, and a switch is providedfor applying to the stator coils voltage of a commercial three-phase ACpower supply with a commercial frequency remaining intact.

Therefore, a drive system of a cage induction motor is provided torestrict a starting current of a three-phase induction motor, to whichthree-phase alternating current with a commercial frequency is supplieddirectly, and generate an adequate starting torque and to exhibit anexcellent steady-state characteristics such as efficiency, power factor,and so on.

In yet a further aspect of the invention, the rotor conductor comprisesa rotor conductor upper portion, which is positioned on a side of anouter periphery of a rotor, and a cross sectional shape of which taperstoward the outer periphery of the rotor, and a rotor conductor lowerportion, which is positioned nearer to a center of the rotor than therotor conductor upper portion, and a cross sectional shape of which hassubstantially the same width as that of a bottom of the rotor conductorupper portion.

In a further aspect of the invention, the rotor conductors comprise arotor conductor upper portion, which is positioned on a side of an outerperiphery of a rotor, and a cross sectional shape of which has atrapezoidal, cross sectional shape, and a rotor conductor lower portion,which is positioned nearer to a center of the rotor than the rotorconductor upper portion, and a cross sectional shape of which hassubstantially the same width as that of a bottom of the rotor conductorupper portion.

In another aspect of the invention, the rotor conductors comprise arotor conductor upper portion, which is positioned on a side of an outerperiphery of a rotor, and a cross sectional shape of which tapers towardthe outer periphery of the rotor, and a rotor conductor lower portion,which is positioned nearer to a center of the rotor than the rotorconductor upper portion, and a cross sectional shape of which is maderectangular to have substantially the same width as that of the rotorconductor upper portion.

Accordingly, both starting characteristics and steady-statecharacteristics are improved.

Other objects and features of the invention will be made apparent fromthe following description of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a cross sectional view showing an essential part of acage induction motor according to an embodiment of the invention and aview showing a configuration of a drive system;

FIG. 2 is a cross sectional view showing an essential part of a rotoraccording to the embodiment of the invention;

FIG. 3 is a graph indicating slip/torque characteristics of theinduction motor according to the embodiment of the invention;

FIG. 4 is a graph indicating starting characteristics and steady-statecharacteristics of the induction motor according to the embodiment ofthe invention;

FIG. 5 is a graph indicating starting characteristics and steady-statecharacteristics of an induction motor according to a further embodimentof the invention;

FIG. 6 is a graph indicating a starting characteristics and asteady-state characteristics of an induction motor according to a stillfurther embodiment of the invention; and

FIG. 7 is a cross sectional view showing an essential part of a rotoraccording to a further embodiment of the invention.

FIG. 8 is a cross sectional view showing an essential part of a rotoraccording to a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described below in detail on the basis ofembodiments as shown.

FIG. 1 includes a cross sectional view showing an axial cross section ofan upper half of a cage induction motor according to an embodiment ofthe invention and a view showing a configuration of a drive system ofthe induction motor. A stator 10 of the cage induction motor comprises astator core 11, a multiplicity of stator slots 12 formed at spaces nearto an inner periphery of the stator core 11, and stator coils 13embedded in the slots 12. The stator core 11 comprises an axiallaminate, both ends of which are secured by core holding members 14.

Meanwhile, a rotor 20 comprises a rotor core 22 laminated on and fittedinto a shaft 21, both ends of the rotor core being secured by coreholding members 23. A multiplicity of slots 24 are formed at spaces nearto an outer periphery of the rotor core 22 to extend radially, and rotorconductors 25 are provided to be embedded into the slots 24. Thereference numeral 26 denotes a short ring.

The invention achieves an improvement in configurations of the rotorslots 24 and the rotor conductors 25 received therein. As shown in anenlarged, cross sectional view taken along the line A-A, with theembodiment, a cross sectional shape of the rotor conductors 25 iscomposed of an upper trapezoidal-shaped portion tapered linearly towardan outer periphery of the rotor and a lower rectangular-shaped portioncontiguous to the upper portion and having substantially the same widthas that of a bottom of the upper portion. This will be described indetail later.

Also, provided as a power-supply unit is a switch 33, by whichthree-phase alternating current received by an incoming panel 32 from acommercial three-phase AC power supply 31 is supplied to the stator coil13 of the three-phase induction motor while a commercial frequencyremains intact (through non-medium of any variable frequency inverter).

FIG. 2 is an enlarged view showing an essential part of the rotor 20,according to the embodiment of the invention, as sectioned in adirection perpendicular to the shaft. A plurality of the rotor slots 24are provided at predetermined spaces in a circumferential direction ofthe rotor core 22 and the rotor conductors 25 are received in the slots24. In the case of a cage induction motor of 10 MW class, the rotorconductors 25 have, for example, a total height h0=60 mm, and a ratioW1/W2=1/3.25 of a width W1 at an upper end thereof and a width W2 at abase thereof. Here, a rotor conductor upper portion 251 positioned nearto the outer periphery of the rotor 20 and having a height dimension h1has a trapezoidal, cross sectional shape linearly tapering toward theouter periphery of the rotor. Meanwhile, a rotor conductor lower portion252 positioned contiguous to the rotor conductor upper portion 251 andnearer to a center of the rotor 20 than the rotor conductor upperportion 251 and having a height dimension h2 has a rectangular, crosssectional shape having a width W2 substantially equal to a bottom of theupper portion 251.

According to the first embodiment, the rotor conductor upper portion 251positioned near to the outer periphery of the rotor 20 has a crosssectional shape linearly tapering toward the outer periphery of therotor 20 and thus being trapezoidal. However, the cross sectional shapeis not necessarily linear but can be curved gently.

With the cage induction motor, current passes through the whole rotorconductors 25 in a steady state while current passes only in thevicinity of outer peripheries of the rotor conductors 25 due toinfluences of the skin effect at the starting. Therefore, by taperingthe width W1 of the rotor conductor upper portion 251, according to theembodiment, near to the outer periphery of the rotor 20, secondaryleakage reactance is increased at the starting and so a starting currentcan be reduced. Also, by tapering the width W1 near to the outerperiphery of the rotor, secondary resistance is increased at thestarting and so a starting torque can be increased. Further, by makingthe width W2 near to the center of the rotor larger than the width W1near to the outer periphery of the rotor, an increase in secondaryleakage reactance and secondary resistance is restricted, so that animprovement in characteristics in a steady state can be achieved.

FIG. 3 is a graph indicating a slip versus torque characteristics of theinduction motor, according to the first embodiment of the invention, incontrast to that of JP-A-7-231630. With a construction of JP-A-7-231630,a starting torque Tst and a maximum torque Tmax are good but a width ofrotor conductors is sharply increased in a stepwise manner and secondaryresistance is sharply reduced. Therefore, a motor torque Tc is reducedaround a middle speed, that is, in the vicinity of slip s=0.5 as shownin the figure. In contrast, with the construction of the rotorconductors 25 according to the embodiment, a width of the rotorconductors 25 is gently increased from W1 to W2, and so it is possibleto obtain a torque characteristics Tm free from torque reduction in thevicinity of a middle speed.

Also, with the construction of JP-A-7-231630, coming-out of rotorconductors due to centrifugal forces is restricted only by the invertedT-shaped steps. Therefore, centrifugal forces concentrate on the stepsand the rotor conductors are in some cases broken by friction. Incontrast, with the rotor conductors 25 according to the embodiment, awhole inclination portion of the upper portion shown in FIG. 2 extendingover the height h1 sustains coming-out of the rotor conductor 25 due tocentrifugal forces. Accordingly, it is possible to relax concentrationof centrifugal forces to prevent breakage of the rotor conductors 25 dueto friction as compared with the construction of JP-A-7-231630.

Further, in the case where a drawn material is used for the rotorconductors 25, the construction of JP-A-7-231630 needs a high techniquefor processing of the rotor conductors since a width of the rotorconductors is sharply changed in a stepwise manner. Meanwhile, the rotorconductors 25 according to the embodiment involve no sharpconfigurational change in that portion thereof, in which a width thereofchanges from W1 to W2, so that drawing of the rotor conductors 25 ismade easy.

Meanwhile, with the rotating electrical machine of“Transformer/induction machine/AC commutator motor”, in which rotorconductors are inclined over a total height thereof to be trapezoidal inshape, the rotor conductors are restricted in height h0 and widths W1,W2 at upper and lower ends thereof due to the trapezoidal shape thereof.Therefore, there is no freedom in design, and starting characteristicsand steady-state characteristics are not appropriately balanced in somecases. In contrast, with the construction of the rotor conductors 25according to the embodiment, it is possible to freely select a height h1for the rotor conductor upper portions 251 and a height h2 for the rotorconductor lower portions 252. Accordingly, there is produced freedom indesign, and starting characteristics and steady-state characteristicsare appropriately balanced with ease.

Hereupon, a rotating electrical machine having the rotor conductors 25according to the invention is effective specifically in large-sized cageinduction motors, in which the switch 33 shown in FIG. 1 leads thecommercial three-phase AC power supply 31 in directly for use while acommercial frequency remains intact. It is thought that large-sized cageinduction motors generally have the same efficiency in rated operationas or higher than that of converter/ inverters. Therefore, it can besaid that the drive system of the cage induction motor, according to theembodiment, leading a commercial three-phase AC power supply in directlyfor use is a motor system having a highest efficiency in rated operationfor use at a constant speed.

Also, a demand for reducing a power supply facility for rotatingelectrical machines in capacity to reduce an initial cost thereof isdeep-rooted. Accordingly, existence of the rotating electrical machineaccording to the invention is significant, which machine does not needany converter/inverter and unites the starting characteristics of lowstarting current and high starting torque and the steady-statecharacteristics of high efficiency and high power factor.

FIG. 4 is a graph indicating starting characteristics and steady-statecharacteristics of a cage induction motor according to an embodiment ofthe invention. Taken as an example are rotor conductors 25 having atotal height h0=60 mm and provided in the cage induction motor of 10 MWclass described above. The graph indicates a starting current Ist, astarting torque Tst, a power factor Pf in a steady state, and efficiencyEff in a steady state in the case where brass is used for the rotorconductors 25 and a height h1 of rotor conductor upper portions 251 isvaried. Based on the characteristics, the rotor conductor upper portions251 in the embodiment of the invention have a height h1=27 mm or more.It is found that when the height h1 is not less than 27 mm, the startingcurrent Ist can be reduced to less than 80% while ensuring a startingtorque Tst not less than that in the case where the height h1 is 0 mm,that is, the rotor conductors are configured to be rectangular over theentire height (left end in the figure). In addition, when the height h1is not more than 27 mm, the starting torque Tst is also reduced thoughthe starting current Ist can be reduced.

Also, as apparent from the figure, it is desired that an upper limit ofthe height h1 of the upper portions 251 of the rotor conductors be notmore than h1=46 mm in order to ensure a power factor 0.975 in a steadystate. This results in ensuring a height h2 of ten and several mm ormore for the lower portions 252 of the rotor conductors 25 and ensuringan efficiency in a steady state.

FIG. 5 is a graph indicating starting characteristics and steady-statecharacteristics of a cage induction motor according to a furtherembodiment of the invention. Likewise, copper is used for rotorconductors 25 having a total height h0=60 mm and provided in a cageinduction motor of 10 MW class. Like that in FIG. 4, the graph indicatesa starting current Ist, a starting torque Tst, a power factor Pf in asteady state, and an efficiency Eff in a steady state in the case wherea height h1 of rotor conductor upper portions 251 is varied. Based onthe characteristics, the rotor conductor upper portions 251 in theembodiment have a height h1=7 mm or more. It is found that when theheight h1 is not less than 7 mm, the starting current Ist can be reducedto less than 90% while ensuring a starting torque Tst not less than thatin the case where the rotor conductors have a height h1=0 mm and areconfigured to be rectangular (left end in the figure). In addition, whenthe height h1 is not more than 7 mm, the starting torque Tst is alsoreduced though the starting current Ist can be further reduced.

Also, as apparent from the figure, it is desired that an upper limit ofthe height h1 of the upper portions 251 of the rotor conductors 25 benot more than 44 mm in order to ensure a power factor of 0.975 in asteady state. This results in ensuring a height of ten and several mm ormore for the lower portions 252 of the rotor conductors 25 and ensuringefficiency in a steady state.

FIG. 6 is a graph indicating starting characteristics and steady-statecharacteristics of a cage induction motor according to a still furtherembodiment of the invention. Copper is used for rotor conductors 25having a total height h0=30 mm and provided in a cage induction motor of100 MW class. Like those in FIGS. 4 and 5, the graph indicates astarting current Ist, a starting torque Tst, a power factor Pf in asteady state, and efficiency Eff in a steady state in the case where aheight h1 of rotor conductor upper portions 251 is varied. Based on thecharacteristics, the rotor conductor upper portions 251 in theembodiment have a height h1=7 mm or more. It is found that when theheight h1 is not less than 7 mm, the starting current Ist can be reducedto less than 90% while ensuring a starting torque Tst not less than thatin the case where the rotor conductors have a height h1=0 mm and areconfigured to be rectangular (left end in the figure). In addition, whenthe height h1 is not more than 7 mm, the starting torque Tst is alsoreduced though the starting current Ist can be reduced.

Also, as apparent from the figure, it is desired that an upper limit ofthe height h1 of the upper portions 251 of the rotor conductors 25 benot more than 17.5 mm in order to ensure a power factor of 0.99 forinduction motors of this class in a steady state. This results in fullyensuring a height 2 for the rotor conductor lower portions 252 to obtaina high efficiency in a steady state.

FIG. 7 is an enlarged view showing an essential part of a rotor 20,according to a further embodiment of the invention, as sectioned in adirection perpendicular to a shaft. A plurality of rotor slots 24 areprovided at predetermined spaces in a circumferential direction of arotor core 22 and rotor conductors 25 are received in the slots 24.

In the embodiment, the rotor conductors 25 comprise a rotor conductorupper portion 2511, which is made of brass and positioned near to anouter periphery of a rotor 20, and a rotor conductor lower portion 2521,which is made of copper to be positioned contiguous to a bottom of therotor conductor upper portion 2511 and near to a center of the rotor.And the rotor conductor upper portion 2511 has a cross sectional shapecontinuously tapering toward to an outer periphery of the rotor 20, andthe rotor conductor lower portion 2521 has a rectangular cross sectionalshape having substantially the same width as that of the bottom of therotor conductor upper portion 2511.

In this manner, the rotor slots 24 and the rotor conductors 25 areshaped in the same manner as in the embodiment shown in FIG. 2, and havethe same dimensions as those illustrated in FIGS. 2 and 4.

As described above, with the cage induction motor, current passesthrough the whole rotor conductors 25 in a steady state while currentpasses only in the vicinity of the outer peripheries of the rotorconductors 25 at the starting due to influences of the skin effect.Therefore, those rotor conductor upper portion 2511, according to theembodiment, which are made of brass to be high in specific resistivity,increase secondary reactance at the starting, thus enabling the startingtorque in the same manner as in the embodiment shown in FIG. 2. Also,the use of copper of low specific resistivity for the rotor conductorlower portions near the center of the rotor restricts loss in the rotorconductor lower portions 2521 in a steady state to be able to achieve ahigher efficiency than that in the embodiment shown in FIG. 2.

The cross sectional shape of the rotor conductor lower portion does nothave to be made rectangular. In FIG. 8, for example, a bottom portion253 of the rotor conductor lower portion 252 may be made half-rounded.

When the rotor conductor upper portion has a trapezoidal, crosssectional shape which tapers toward the outer periphery of the rotor,and the width W3 of the cross sectional shape of the rotor conductorupper portion is substantially same as the width W4 of the crosssectional shape of the rotor conductor lower portion, both startingcharacteristics and steady-state characteristics are improved.

According to the invention, it is possible to provide a rotatingelectrical machine having characteristics of low starting current andhigh starting torque and a good balance between a startingcharacteristics and steady-state characteristics.

Also, it is possible to provide a drive system of a cage inductionmotor, in which alternating current with a commercial frequencyremaining intact is supplied directly to the induction motor, andachieves a characteristics of low starting current and high startingtorque and a characteristics of high efficiency and high power factor.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An electrical motor comprising a rotor core, a slot radially providedin a circumferential direction of the rotor core, and a rotor conductorreceived in the slot, wherein the rotor conductor comprises: a rotorconductor upper portion, which is positioned on a side of an outerperiphery of a rotor, and a cross sectional shape of which tapers towardthe outer periphery of the rotor, and a rotor conductor lower portion,which is positioned nearer to a center of the rotor than the rotorconductor upper portion, and a cross sectional shape of which hassubstantially the same width as that of a bottom of the rotor conductorupper portion.
 2. The electrical motor according to claim 1, wherein aratio of a height of the rotor conductor upper portion to a total heightof the rotor conductor is from 27/60 to 46/60.
 3. The electrical motoraccording to claim 1, wherein a ratio of a height of the rotor conductorupper portion to a total height of the rotor conductor is from 7/60 to44/60.
 4. The electrical motor according to claim 1, wherein a ratio ofa height of the rotor conductor upper portion to a total height of therotor conductor is from 7/60 to 17.5/60.
 5. An electrical motorcomprising a rotor core, a slot radially provided in a circumferentialdirection of the rotor core, and a rotor conductor received in the slot,wherein the rotor conductor comprises: a rotor conductor upper portion,which is positioned on a side of an outer periphery of a rotor, and across sectional shape of which has a trapezoidal, cross sectional shape,and a rotor conductor lower portion, which is positioned nearer to acenter of the rotor than the rotor conductor upper portion, and a crosssectional shape of which has substantially the same width as that of abottom of the rotor conductor upper portion.
 6. The electrical motoraccording to claim 5, wherein a ratio of a height of the rotor conductorupper portion to a total height of the rotor conductor is from 27/60 to46/60.
 7. The electrical motor according to claim 5, wherein a ratio ofa height of the rotor conductor upper portion to a total height of therotor conductor is from 7/60 to 44/60.
 8. The electrical motor accordingto claim 5, wherein a ratio of a height of the rotor conductor upperportion to a total height of the rotor conductor is from 7/60 to17.5/60.
 9. An electrical motor comprising a rotor core, a slot radiallyprovided in a circumferential direction of the rotor core, and a rotorconductor received in the slot, wherein the rotor conductor comprises: arotor conductor upper portion, which is positioned on a side of an outerperiphery of a rotor, and a cross sectional shape of which tapers towardthe outer periphery of the rotor, and a rotor conductor lower portion,which is positioned nearer to a center of the rotor than the rotorconductor upper portion, and a cross sectional shape of which is maderectangular to have substantially the same width as that of the rotorconductor upper portion.
 10. The electrical motor according to claim 9,wherein a ratio of a height of the rotor conductor upper portion to atotal height of the rotor conductor is from 27/60 to 46/60.
 11. Theelectrical motor according to claim 9, wherein a ratio of a height ofthe rotor conductor upper portion to a total height of the rotorconductor is from 7/60 to 44/60.
 12. The electrical motor according toclaim 9, wherein a ratio of a height of the rotor conductor upperportion to a total height of the rotor conductor is from 7/60 to17.5/60.